Late Innate Immune Responses Flashcards
Revision
What is the complement system and what happens when it is activated?
Low levels of inactive complement system proteins are found in extracellular fluids normally.
The complement system, when activated, creates a cascade of chemical reactions that promotes:
- Opsonization of pathogens
- Direct pathogen killing
- Acute inflammation
- Leukocyte recruitment
What is the overview of the complement system?
C3 -> C3b + C3a (activation of downstream complement proteins) - Classical pathway - Mannose - binding lectin pathway - Alternative pathway (C3 is an acute Phase Protein)
What is the process of the Mannose-binding Lectin (MBL) Pathway?
Selective activation of the MBL pathway arises because mannose expression is unique to certain pathogens. It is not expressed on human cells.
- MBL is an acute Phase protein.
Low level, spontaneous cleavage of C3 into C3b and C3a does occur, even in the absence of pathogens. Luckily, C3b is unstable and rapidly degraded unless it binds to cell surfaces:
- Binding of C3b to a pathogen stabilises C3b allowing it to activate downstream events of the complement system.
- Human cells expression special inhibitory proteins that prevent C3b from activating downstream events and ensures the response is specific to an infection setting.
Note: Once C3b is generated an amplification loop stimulates more and more C3 cleavage (via the alternative pathway).
What is the process of the downstream Complement Pathway?
Active C3b associates with other complement system proteins producing a C5 convertase, which cleaves inactive C5 into active C5a and C5b.
Active C5b goes onto associate with other complement system proteins to produce a pore-forming channel which inserts into the pathogen membrane/ cell wall.
- This is the MAC (membrane attack complex)
- Extracellular salts and water enter the pathogen via this pore, causing the pathogen to swell and burst.
C3a and C5a are known as “anaphylatoxins”.
- THey promote changes in the local vasculature, acute inflammation and leukocyte recruitment by:
- Activating mast cels (which then degranulate, releasing histamine and other pro-inflammatory mediators)
- By acting directly on local blood vessels.
C3b is a very powerful opsonin
- Remember: Opsonins are soluble factors that bind to pathogens and enhance phagocytosis.
What is acute inflammation, when does it occur and why is it important?
Acute inflammation is the tissue response to infection (or injury)
Onset: immediate -> days
This is an important response to cellular injury or infection. It has both a rapid innate phase as well as a prolonged phase that is an important component of acquired immunity. Immediately after injury or infection a number of proteins are released. These lead to the physiological characteristics if inflammation: swelling, redness, heat and last but not least, pain.
A key response to local changes to the vasculature is th recruitment of circulating innate immune cells (such as neutrophils and monocytes) into the infected/inflamed tissue.
What does healthy tissue contain?
Healthy tissues: - No inflammatory mediators - Normal vasculature - Circulating neutrophils Infalmmatory Promotes: - Vasculature changes - Recruitment and activation of neutrohpils (transendothelial migration)
The process of transendothelial migration can be divided up into a number of broad steps. What are these steps and what processes occur in each step?
- Margination of neutrophils to the endothelium near sites of tissues damage/infection.
- Normally blood cells flow in the centre of the lumen (axial flow), the are adjacent to the blood vessel wall carries only plasma.
- Loss of intravascular fluid and increased plasma viscosity slows blood flow allowing neutrophils to move within the plasmatic zone. This is margination.
- Margination of neutrophils, occurs only in post-capillary venules. - Binding of neutrophils to adhesion molecules (selections, ICAM-1) on the endothelial cells.
- This process is receptor-mediated. Pro-inflammatory mediators, such as histamine and TNFalpha, promote the immediate expression of adhesion molecues (known as Selectins) on endothelial cell surfaces.
- These adhesion molecules bind eakly to carbohydrate ligands expressed on the surface of neutrphila and causes them to “roll” along the endothelial surface as bonds are made and broken.
- THese inflammatory mediators also include the expression of adhesion molecules such as ICAM-1 on endothelial cells, which bind to specific receptors (such as LFA-1) which are present on the surface of Neutrophils.
- Chemokines (produced by injured cells and innate immune cells) are needed to full activate LFA-1 receptors so that they can bind firmly to ICAM adhesion molecules and stably adhere to the endothelial surface. - Migration of neutrophils acress the endothelium, via the pocess of diapdedis.
- Chemokines and microbial-derived chemical gradients stimulate the adhered neutrophils to move between endothelial cells and pass through the basement membrane into infected/inflamed tissues. - Movement of neutrophils within the tissue via chemotaxis.
- Neutrophils reaching the tissue interstitium bind to extracellular matrix proteins via expressed integrins to prevent their loss from the site. Chemokines and microbial products cause the Neutrophils to move along a chemotactic gradient towards the source of inflammation/infection. - Activation of neutrophil by PAMPs and TNFaplha
- Neutrophil activation is enhanced in the presence of pro-inflammatory mediators such as TNFalpha.
What do Neutrophils do?
Neutrophils are killing mechanisms: - Phagocytosis - Degranulation - NETs (Neutrophil extracellular traps (NETs) are networks of extracellular fibres, primarily composed of DNA from neutrophils, which bind pathogens. NETs allow neutrophils to kill extracellular pathogens while minimizing damage to the host cells)
What is the role of Phagocytosis and what are the two different mechanisms used to kill internalised pathogens?
In infected tissues, pathogens release chemical signals that attract neutrophils.
Neutrophils use “Pattern Recognition Receptors” to bind to and pagocytose these pathogens.
Kill internalised pathogens via two distinct mechanisms:
Phagolysossomal killing
ROS-dependent killing
(Similar to macrophage-mediated phagocytosis and killing except that unlike macrophages, Neutrophils are better killer - they have a second way to kill internalised pathogens via a ROS-dependent mechanism.
What is the process of degranulation?
After phagocytosis and killing of internalised pathogens, Neutrophils can eliminate nearby extracellular pathogens by releasing residual enzymes/toxins.
But this can lead to tissue damage and (potentially) systemic inflammation.
- Limited by increasing expression of Proteinase inhibitors (which are produced as part of the Acute Phase Response)
Release of anti-bacterial proteins from neutrophil granules directly into the extracellular milieu
- Diect killing of extracellular pathogens bacteria and fungi
- Tissue damage and (potentially) systemic inflammation.
In a penetrating injury and infection of the foot what would the local and systemic clinical features be?
Systemic Resonses: Fever Bone marrow: increased production and mobilisation of neutrophils. Liver: increased synthesis of acute phase proteins. Local: Local cellulitis Pain Redness Swelling
What is the role and process of the acute phase response?
This systemic response involves changes in the plasma concentrations of specific proteins in response to inflammation:
Driven by pr-inflammatory mediators released by activatied macrophages.
Mediated by liver hepatocytes which produce a variety of Acute Phase proteins.
C3 and MBL
- Complement system proteins
C reactive protein (CRP)
- Primes certain bacteria for destruction by the complement system
- Has a prognostic role (severity, duration of inflammation)
CRP = prognostic
- very rapidly increased during inflammation (up to 30 thousand fold)
- very short 1/2 life also, so rapidly disappears once inflammation is resolved.
What is the C reactive Protein?
A major acute phase protein in humans
Used as a marker for inflammation
1. Diagnosis/prognosis
What about intracellular pathogens? e.g. viruses
Viruses are too small to be degranulated or phagocytosed.
What are the roles of Natural-killer (NK) cells?
NK cells specifically kill infected cells and abnormal cancer cells. They ignore normal healthy tissues and cells.
Natural-killer (NK) cells are lymphocytes that can recognize and destroy viral or cancerous cell.
Recruited into infected/inflamed tissues in a very similar manner to neutrophils (but are longer-lived).
NK cells contact other cells. If the other cell appears to be abnormal (i.e. virally infected or cencerous) then the NK cell releases cytotoxic molecules that cause the abnormal cell to undergo apoptosis.
NK cells respond to levels of class I MHC (major histocompatibility antigen) molecule on cell surface.
Reduced levels of HC class I occurs in virally infected and cancerous cells, result in killing.
